I foresee one of two issues. Either the box will be tight enough fit to keep the free play down so that the rebate is accurate but may be so tight as to make it hard to push through or the fit will be loose enough to allow you to push freely but with too much free play to make a uniform rebate. Ideally you would want the board to register firmly against two sides with feather boards or spring loaded rollers to keep it that way. You might find it easier by hand. When doing jobs like that you can use a double edge guide system which traps the board between them but a single edge guide works too. I just find that I usually need to make 2 or more passes to make sure I’ve routed it evenly. In you click on my uploads I made a home made edge guide to add to a factory one. It will be a few pages back and I think I posted it in a thread entitled Shop Hacks.